Skeletal muscle tissue is often used to provide cardiac assistance. Such systems which utilize skeletal muscle tissue may be seen in U.S. Pat. No. 4,411,268, issued to Cox, and U.S. Pat. No. 4,813,952, issued to A. Khalafalla, and U.S. Pat. No. 4,735,205 all assigned to Medtronic, Inc., and incorporated herein by reference.
Such systems use a patient's own muscle tissue in conjunction with a implantable pulse generator to provide cardiac assistance. In comparison to presently available cardiac assist systems using wholly artificial structures, systems using a patient's skeletal muscle are extremely compact and energy efficient. Such cardiac assist systems, however, are not without limitations. One problem presented by the use of skeletal muscle power for cardiac assistance is the application of electrical stimulation signals to cause skeletal muscle contraction.
The electrical connection between an implantable pulse generator and the desired skeletal muscle is accomplished through a lead. Generally speaking a lead is a wire insulated along its length and having an electrode at one end and connectable to a pulse generator at its other end. Through a lead then electrical signal may be communicated to and from skeletal muscle tissue.
The earliest skeletal muscle powered cardiac assist systems used screw-in type leads for skeletal muscle stimulation. A major improvement to these leads is found in the use of steroid eluting pacing leads. U.S. Pat. No. 4,711,251 issued to Stokes, and assigned to Medtronic, Inc. teaches the use of an endocardial pacing lead having steroid drug embedded in the distal tip. This embedded steroid drug treats the heart tissue immediately in contact with the pacing electrode. U.S. Pat. Nos. 4,506,680; 4,577,642; and 4,606,118 teach similar endocardial leads, all of which treat the electrode contact area with asteroid. United States Statutory Invention Registration No. H356 discloses an endocardial pacing lead suitable for epicardial insertion which elutes a steroid drug from the electrode.
A further improvement in intramuscular lead technology arose with the adaptation of heart wire technology for chronic pacing use. Typically such leads are constructed as follows: A connector assembly has a coiled connector attached thereto. The coiled connector is insulated along a part of its length while a suture runs throughout its inner lumen, from the connector assembly to an end. At the end of the suture a helical portion is formed, and a needle is attached to the end of the suture. The suture material is treated with asteroid drug, such as a glucocorticosteroid, along its entire length. Additional drugs which may be imbedded within strand include antibiotics. Upon chronic implantation, the steroid drug is eluted from the suture material, thus treating possible tissue inflammation or damage caused by the implantation procedure or subsequent irritation.
One drawback to such a lead as presently configured is found at the conductor coil-suture interface. In designs presently in use the conductor coils are attached to the end of suture by a crimp sleeve. In such a manner a tip electrode is formed. Because the suture is used to pull the electrode coil through muscle tissue during implantation, crimp sleeve used to form tip electrode, which has a larger diameter than either suture or electrode coil, creates friction. Such friction creates difficulties to the physician during implantation. For this reason a flexible, specifically designed lead having a relatively slender dimension at the conductor coil-suture interface is desired.